Proton conducting membranes and fuel cells are expected to play a very important role in the future of power generation. Some examples of fuel cells include solid oxide fuel cells, polymer electrolyte membrane fuel cells, and molten carbonate and phosphoric acid fuel cells. The cost of solid oxide fuel cells is extremely high due to their elevated operating temperature (>600° C.), which requires the use of expensive materials able to withstand such high temperatures. Polymer electrolyte membrane fuel cells operate below 100° C., however require the use of platinum catalysts that are both expensive and easily contaminated. Operation below 100° C. also produces liquid water as a byproduct that severely limits fuel cell performance as it impedes gas diffusion to the charge transfer interface. Therefore the discovery of proton conductors capable of operating between 100° C.-600° C. has been an important goal in fuel cell research. FIG. 1 illustrates some available proton conductors and their conductivity plotted versus temperature. A gap in the availability of materials that conduct protons at temperatures of from about 100° C.-600° C. is noticeable.
Oxoacids and their salts have been known to exhibit anhydrous protonic conduction above the boiling point of water. However, most oxoacids have very low conductivities. One exception is CsHSO4, which has a conductivity of 10−3-10−2 S/cm, where S means Siemens, at a temperature of 141° C. Although the use of a CsHSO4 electrolyte in a fuel cell was demonstrated, the material is water soluble, is easily reduced under hydrogen conditions, and thus unstable under normal fuel cell operating conditions. Phosphates, for example CsH2PO4, have been used as electrolytes and show good stability up to 250° C. in humid conditions. However, the conductivity of these materials is at least an order of magnitude too low with a very narrow temperature range of operation. In addition, phosphates such as CsH2PO4 cannot operate without the presence of water.
Recently it was reported that SnP2O7 and Sn0.9In0.1P2O7 have excellent protonic conductivity (10−1 S/cm) at 80-300° C., and exhibit a gradual increase in conductivity from 80° C. to 300° C. In addition, it was reported that these materials are stable and highly conductive in the anhydrous state, and do not appear to exhibit a superprotonic transition. However, methods for preparing SnP2O7 have included mixing a Sn-containing salt or oxide with phosphoric acid and calcining (or heating) the mixture at a high temperatures (≧600° C.), which forms a SnP2O7 phase by evaporation of the excess P2O5. In this process it is difficult to control and reproduce the Sn/P ratio, with the only control being the temperature and time of calcination. A currently accepted method of preparing Sn0.9In0.1P2O7 includes evaporating a mixture with excess phosphorous, carefully controlling a variety of reaction conditions such as sample size, crucible shape and heating and cooling rates, and consistently selecting only the portion of sample in the middle of the evaporated product as a means of controlling the consistency of the stoichiometry of the tin and indium.
Thus, a need exists a method of making materials that have high proton conductivity at a temperature range of from about 100° C.-600° C., which do not produce water as a byproduct during operation, and which allows improved reproducibility of the Sn/P ratio and the Sn/In ratio.